Dense patterns are understood to mean patterns some elements of which are very narrow and separated by intervals that are very narrow. With photolithography, a sensitive layer is exposed, for example through a mask, for example to an ultraviolet beam, typically at a wavelength of 193 nanometers. The density of the patterns, in other words the resolution that can be achieved in the etching of the pattern, is linked to several parameters but it firstly depends on the wavelength used; the shorter it is the higher is the density. The density of the patterns can be increased by using extreme ultraviolet (notably at 13.5 nanometers); it can also be increased by using an exposure to an electron beam of very small diameter, but the process of etching is much slower because the writing of a pattern is carried out point by point and not through a global mask defining the patterns; furthermore, narrow electron beams can generate phenomena of dispersion and back-scattering of electrons to the detriment of the resolution.
In the field of memories, in order to increase the possible density of the lines needed in order to fabricate these memories, a known solution is to carry out the following steps:                formation of a sacrificial layer on a substrate,        etching of the sacrificial layer according to first strips,        formation of spacers along all the edges of the elements of the sacrificial layer thus etched;        removal of the sacrificial layer so as to leave only the spacers remaining forming the dense lines sought.        
This method is sometimes called pitch-multiplication lithography because it results in a distance between lines smaller (practically two times smaller) than that allowed by a direct lithography.